Release properties with respect to pressure-sensitive adhesive materials have hitherto been conferred by forming a cured coating of silicone composition on the surface of a sheet-like substrate of paper, plastic or the like.
Known methods of forming cured coatings of silicone compositions on substrate surfaces include (1) to (3) below.
(1) The method of forming a release coating on a substrate face by addition reacting an alkenyl group-containing organopolysiloxane with an organohydrogenpolysiloxane using a platinum compound as the catalyst (Patent Document 1: JP-A S47-32072).
(2) The method of forming a release coating by condensation reacting a hydroxyl group or alkoxy group-containing organopolysiloxane with an organohydrogenpolysiloxane using an organic acid metal salt such as an organotin compound as the catalyst (Patent Document 2: JP-B, S35-13709).(3) The method of forming a release coating by using ultraviolet radiation or electron beams to induce the radical polymerization of an acrylic group-containing organopolysiloxane with a photoreaction initiator (Patent Document 3: JP-A S54-162787).)
Of the foregoing methods of forming an organopolysiloxane cured coating on a substrate surface, the method of forming a release coating via an addition reaction is widely used because the curability is excellent and this approach is able to address a variety of release property requirements from low-speed release to high-speed release.
Organopolysiloxane compositions for release liners are generally required to have various release forces according to the intended purpose. Compositions in which an alkenyl group-containing MQ resin (the M unit signifying an R3SiO1/2 unit (where R is a monovalent hydrocarbon group), and the Q unit signifying a SiO4/2 unit) have been added are widely used for applications requiring a tight release force.
However, the release-tightening effect of alkenyl group-containing MQ resins is not all that high; even when a large amount is added, the target release force is sometimes not achieved. Because such alkenyl group-containing MQ resins are expensive, adding a large amount is undesirable in terms of cost. Hence, there exists a desire for compositions that, in a small amount, have an excellent release-tightening effect. Also, alkenyl group-containing MQ resins are known to have a tendency for the release force to decrease with the passage of time compared with the release force immediately after curing. Accordingly, there is a desire for compositions that do not undergo a change in the release force over time.
JP-B H05-53183 (Patent Document 4) uses an alkenyl group-containing MQ resin and an alkenyl group-free MQ resin together within an organopolysiloxane composition for a release liner. Although this does lower the change in release force over time, the release-tightening effect is inadequate.
JP No. 2750896 (Patent Document 5), by including an alkenyl group-containing resin within a solvent-based addition-curable organopolysiloxane composition for a release liner, achieves low-temperature curability and a release force that undergoes little change over time. However, release-tightening is not the object of this art, and the release force is not tight.
JP No. 2742835 (Patent Document 6) adds, to an addition reaction-type organopolysiloxane composition, the equilibration reaction product of a vinyl group-containing organopolysiloxane and an organopolysiloxane of the formula (R3SiO1/2)a(R2SiO)b(RSiO3/2)c(SiO4/2)d(wherein R is a monovalent hydrocarbon group and the subscripts a, b, c and d each represent the molar fractions of the respective siloxane units, with a=0.1 to 0.6, b=0 to 0.45, c=0 to 0.3, and d=0.3 to 2.0), and can be thought of as an MQ resin having a vinyl-terminated siloxane bonded thereto.
However, no mention is made of the optimal range in the structure. In the examples, use is made of a tight release additive composition obtained by heat-treating for 5 hours at 100° C. and using potassium hydroxide as the catalyst: 40 parts by weight of polydimethylsiloxane containing vinyl groups at both ends and having a degree of polymerization of about 8,000, and 60 parts by weight of a 30 wt % toluene solution of organopolysiloxane consisting of (R3SiO1/2)a units (M units) and (SiO4/2)d units (Q units) in a molar ratio of from 0.8 to 1.0. However, because this composition has a degree of polymerization that is too high, it cannot be used unless it is diluted in an organic solvent. Also, the release-tightening effect obtained with the addition of 10 parts by weight of the tight release additive is about 2 to 3.8-fold, which remains inadequate.
The following ways of carrying out release-tightening by adding an organopolysiloxane resin having tackiness to an addition reaction-curable organopolysiloxane composition have been disclosed.
JP-B H06-86582 (Patent Document 7) describes a tacky organopolysiloxane protective coating agent that includes the product obtained by partial dehydration condensation of a curable silicone rubber, a dihydroxy-terminated organopolysiloxane and an MQ unit-containing silicone resin. Because the silicone rubber and the silicone resin have a high viscosity or are solid, a solvent is required. This product is intended for use as a pressure-sensitive adhesive; no mention is made of its effects as a tight release additive for a release liner. Also, although the scope of the claims is broad, the body of the specification and the examples section do not mention anything other than the compounding ratio of the silicone rubber and the MQ unit-containing silicone resin.
JP-A H10-110156 (Patent Document 8) relates to an organopolysiloxane-based pressure-sensitive adhesive that uses as the base resin a mixture or partial condensation product of vinyl crude rubber and an MQ resin. Because it uses a silicone rubber, a solvent is required. No mention is made of advantageous effects as a tight release additive for a release liner.
JP-A 2010-37557 (Patent Document 9) relates to a release adjusting agent made up of an organopolysiloxane resin-organopolysiloxane condensation reaction product obtained by the condensation reaction of (a1) 100 parts b weight of an MQ resin having a molar ratio of M units to Q units of from 0.6 to 1.0 and a content of hydroxyl groups or alkoxy groups in the range of 0.3 to 2.0 wt % with (a2) from 20 to 150 parts by weight of a hydroxyl group or alkoxy group-containing acyclic polydiorganosiloxane having an average degree of polymerization of from 100 to 1,000. Relative to a comparative example in which the condensation reaction product has not been added, the release force in the examples rises only from 1.4 to 2.2-fold at a low speed (0.3 m/min), and so a sufficient release tightening effect is not obtained. The reason is presumably that the range of 0.3 to 2.0 wt % in the content of hydroxyl groups or alkoxy groups in the component (a1) used here is low, resulting in few reactive sites with component (a2), as a result of which the product does not have a structure that is sufficiently crosslinked by condensation.
As noted above, even today, resin compositions for a release liner that are made of organic resins such as polypropylene, amino alkyd, acrylic or polyisocyanate resins rather than silicone resins are often used in applications requiring a tight release force, such as casting paper (see Patent Document 10: JP-B S57-48013).
Because these resins have poor release properties, many improvements utilizing silicones have been proposed, including release agent composition containing silicone-modified alkyd resins (see Patent Document 11: JP-B S58-53680), silicone-modified acrylic resins (see Patent Document 12: JP-B S61-13507), silicone-modified alkyd resins (see Patent Document 13: JP-B H04-20954), and amino resins and silicone resins (see Patent Document 14: JP-A 2000-95929).
Owing to such numerous improvements, excellent organic resin-based tight release compositions have come to be supplied. Yet, even now, when these compositions are coated to form a release layer, there remains a need for curing and drying temperatures of 150° C. or more. This heat damages the surface of the release liner, lowering its aesthetic appearance, and also increases energy consumption, running up production costs. Hence, there exists a desire for compositions which can be cured at lower temperatures.
Recently in particular, film substrates have come to be commonly used as release materials in production processes. For example, Patent Documents 15 to 17 (JP-A H11-300894, JP No. 2932911 and JP No. 3459722) relating to the production of ceramic green sheet for use in multilayer ceramic substrates and multilayer ceramic capacitors have been disclosed.
Looking at other applications in which release films can be used, release films wherein the substrate is primarily a polyester film are used for protecting pressure-sensitive adhesive layers in, for example, liquid crystal polarizers and waveplates. In keeping with the demand for larger screens and high-definition picture quality, to achieve the processing accuracy required, there is a desire for high accuracy also in the shape of the release film. Here too, as mentioned above, heating during formation of the release layer is a factor that lowers accuracy due to deformation and thus presents a problem. Accordingly, there is a desire for release agents that can be cured at lower temperatures.
In order to resolve these problems, there exists a desire for the development of a tight release control agent that can make the release force tighter than that of MQ resin and other silicone resin-based tight release control agents, and that can be used by being compounded within an addition-curable silicone composition having excellent curability. However, in methods for tightening release by compounding, within an addition-curable silicone composition, an organic resin-based tight release composition that is outstanding in terms of its tight release force, a number of problems arise, such as a poor cure due to inhibition of the addition curing reaction, a decrease in subsequent adhesion, the inability to obtain a sufficient release-tightening effect and, owing to inadequate compatibility, poor coating appearance and anchorage. Even when attempts are made to remedy this by using a silicone-modified organic resin-based tight release composition, sufficient improvement in compatibility and the suppression of cure inhibition cannot be achieved, and the release-tightening effect by organic-modified silicones is small. For this and other reasons, no practical approach exists at present.